In strip processing lines heretofore, the undesired curvature in the strip has been removed by leveling mechanisms of stretches of these lines in which the strip is brought to a relatively high degree of tension between sets of rollers which frictionally engage the strip, generally upstream and downstream bridles. The upstream bridle is, in effect, a braking set of rollers which retards advance of the strip while the downstream bridle is a set of driven rollers which draws the strip along the path. The conjoint action of the upstream and downstream bridles provides a tension along the path which can eliminate to a certain extent longitudinal curvature by stretch leveling. Between the bridles, a reverse bending set of rollers may be provided to effect stretch-bend leveling. In the strip processing field simple bend leveling or roller leveling may also be used, the strip being bent by rollers which penetrate into the path of the strip.
The curvatures which are removed in the leveling operation may include transverse curvature (cross bowing) and residual longitudinal curvature which is noticeable in sheets cut from the strip and is referred to generally as coil set since that longitudinal curvature tends to be associated with the curvature which remains in the strip after it is unwound from a coil. Both the coil set and the cross bowing can give rise to problems in the processing of the strip in the strip processing line or in fabrication lines to which the strip may be fed for production of products from the metal strip.
To avoid these problems, it is known to reduce the cross bowing in association with stretch bend leveling by adjusting the depth of penetration of stretch bending rolls, thereby compensating for the cross bowing by adjusting the penetration depth of a stretch bending roller. Here, however, the attempt at correction is applied to the strip at a region of maximum tension, i.e. the tension at which the stretch bend leveling occurs.
The strip then passes into a section of the line at which the tension is reduced, i.e. a section beyond the last tension roller or the downstream bridle where the cross bowing is measured by sensors to produce the setting signal for adjusting the penetration depth of the adjustable roller of the stretch bend leveler (see U.S. Pat. No. 4,457,149).
It has been found, with this process that surface defects can result, especially in very thin strip or strip which is sensitive to the formation of surface defects utilizing this approach. In addition, there appears to be a nonuniform effect over the width of the strip so that adjustment of the stretch bending roller does not give rise to uniform elimination of the cross bowing across the width of the strip. This has been found to be a consequence of the multiaxial stresses applied at the high tensions to which the strip may be subject in the stretching section of the path. Furthermore, there is a reduction of width because of the stretch phenomenon over this high tension region.
It is especially disadvantageous with this earlier system that a dead zone or response lag exists between the measurement and correction in this earlier system which makes it impossible to totally eliminate cross bowing and is due to fluctuations in the strip tension, fluctuations in the strip thickness, variations in the elastic limit or yield point or tensile strength of the strip.
As a consequence, segments of the strip can arise which have unsatisfactory levels of cross bowing and which must be cut out of the strip as reject portions. This is costly to production and expensive to carry out.
It is also known to provide strip processing lines which include stretch leveling portions and in which the cross bowing following the stretch leveling is effected over a section of the strip processing line with substantially reduced strip tension by comparison with that which prevails in the stretch leveling section. The cross bowing is here eliminated by a roller bending operation with correcting rollers which have fixed diameter, fixed contact angles with the strip and hence fixed penetration into the path of the strip. The drawback with this system is that the system is sensitive to the choice of the roll diameters, the contact angles of the strip around the periphery of the correcting rollers and the like and these values are not optimal for all metal strip and require selection based upon such parameters as strip tension, strip thickness and strip tensile strength. The setup of the apparatus is time consuming and generally the apparatus cannot readily be reset to take into consideration these parameters except at very high cost and in a time consuming matter.